When a powder fuel such as a fine coal powder is burnt, a cylindrical burning apparatus for the fine coal powder as disclosed in Japanese Examined Patent Publication No. 57-35368 can be used. In the burning apparatus, a plurality of inner primary air-ejection openings are arranged in the center portion of the apparatus, a plurality (four to eight) of fine coal powder-ejection openings for ejecting a mixture of the fine coal powder and air for conveying the coal powder are arranged around the inner primary air-ejection openings and are separated from each other by partitions, and further an outer circumferential primary air-ejection slit having an annular cross-sectional profile is arranged around the fine coal powder-ejection openings. In this apparatus, the fine coal powder is ejected in the form of four to eight ejection streams through the ejection openings separated from each other, and a plurality of inner primary air-ejection straight streams and an annular primary air ejection straight stream are ejected in such a manner that the fine coal powder ejection streams are interposed between the inner primary air-ejection streams and the annular primary air ejection stream. Since the flow speed of the fine coal powder ejection streams is lower than that of the inner and outer primary air ejection straight streams, the fine coal powder-ejection streams are accelerated by the inner and outer primary air-ejection straight streams and the fine coal powder is blown away far. During the above-mentioned ejection, high temperature secondary air is introduced from a product-cooling apparatus arranged downstream of the burning chamber into the burning chamber, passes through gaps of the outer primary air-ejection straight stream, enters inside of the outer primary air-ejection straight stream, and is sucked and diffused into the fine coal powder-ejection streams, to burn the fine coal powder.
Also, the burner for burning a fine particulate solid fuel as disclosed in Japanese Examined Patent Publication No. 2-22,289 is provided with a plurality of inner primary air ejection openings arranged in an annular form in the center portion of the burner and separated from each other through partitions, a plurality of fine particulate solid fuel/conveying air-ejection openings arranged in an annular form around the inner primary air-ejection openings, and outer primary air-ejection opening formed in an annular form around the above-mentioned fine particulate solid fuel/conveying air-ejection openings. In the burner, the flow resistances of the fine particulate solid fuel at the ejection end surfaces are made different from each other, and the distribution density of the fine particulate solid fuel is made uneven, to thereby increase the combustion speed and form a short flame.
Where a powder fuel and primary air are ejected, and high temperature secondary air is mixed into the ejected powder fuel and primary air streams to burn the powder fuel, generally, the combustion of the powder fuel is effected by the total primary air amount and the secondary air in an amount corresponding to the difference between the theoretical combustion air amount and the total primary air amount. In this case, the temperature of the primary air is 60 to 80.degree. C. and the temperature of the secondary air is 800 to 1,000.degree. C. Therefore, the merits of the combustion depend on the primary air ratio (which refers to a ratio of the total primary air amount to the theoretical combustion air amount), and the lower the primary air ratio, the better the combustion.
However, when the primary air ratio is decreased to promote the combustion, the flow speed of the primary air ejection streams is decreased accordingly, the mixing of the secondary air into the combustion mixture becomes insufficient, and thus the above-mentioned decreases causes a disadvantage in that the burning velocity of the powder fuel decreases, the fire point temperature decreases, and incomplete combustion of the fine particulate coal occurs. For these reasons, in the conventional apparatus and method for burning the powder fuel, the primary air ratio is generally, about 20 to 25% and it is difficult to practically use a primary air ratio lower than the above-mentioned level.
Also, in the conventional apparatus and method of burning the powder fuel, it is possible, to a certain extent to adjust the position of fire point by controlling the ratio in flow speed of the inner primary air-ejection straight streams to the inner primary air-ejection turning streams. However, in practice, the above-mentioned control of one burner is difficult. It is necessary to change the design of the inner primary air straight stream-ejection openings and the inner primary air turning stream-ejection opening, in response to the performance of the rotary kiln. Also, in this case, when the inner primary straight air streams are too strong, the resultant burning flame is in the form of a narrow angle long flame, the fire point temperature is insufficient. Also, when the inner primary air turning streams are too strong, the resultant burning flame is in the form of a wide angle short flame. In this case, while the fire point temperature is high, the angle of the flame is too wide and thus the furnace wall is greatly damaged. In a worst case scenario, the furnace wall is damaged.
Also, when a liquid fuel is used, in an apparatus and method for burning a liquid fuel in which the liquid fuel is sprayed into a combustion furnace, the sprayed liquid fuel is mixed with primary air, and further with high temperature secondary air, and is burnt. In this case, the combustion of a combustible substance in the liquid fuel is effected in response to the total primary air amount mixed with the liquid fuel and to the secondary air amount corresponding to the difference between the theoretical combustion air amount and the total primary air amount. Usually, the temperature of the primary air is 60 to 80.degree. C. and the temperature of the secondary air is 800 to 1,000.degree. C. Therefore, the merits of the combustion vary in response to the primary air ratio (which refers to a ratio of the total primary air amount to the theoretical combustion air amount. The smaller the primary air ratio, the higher the temperature of air used for the combustion, and as a result, the burning temperature increases and the fire point temperature rises, and thus good burning occurs.
However, when the primary air amount is decreased to make the burning conditions better, disadvantages such as the primary air-ejection stream velocity decreases, the mixing of secondary air become insufficient, the fire point temperature decreases and the liquid fuel is incompletely burnt, occur. For these reasons, when C heavy oil is used as a fuel in the conventional apparatus and method of burning the liquid fuel, the primary air ratio is controlled to about 12 to 15%. When the primary air ratio is further decreased below the above-mentioned level, good combustion of the liquid fuel is difficult in practice.
In the conventional apparatus and method of burning the liquid fuel, it is difficult to adjust the position of the fire point by controlling the flow velocity ratio of the liquid fuel streams sprayed into a combustion furnace to the primary air-ejection streams concurrently formed with the liquid fuel streams. Therefore, the combustion flame formed in the combustion furnace is in a narrow angle long flame form wherein the fire point temperature may not be sufficiently high, or in a wide angle short flame form in which the fire point temperature is sufficiently high, while the flame spreads too widely and thus the furnace wall is greatly damaged. In a worst case, the furnace wall is damaged.
Further, where a powder fuel and a liquid fuel are employed together, an apparatus and method for burning the powder fuel and the liquid fuel is known. In the apparatus and method, the powder fuel and the liquid fuel are ejected together with primary air and are further mixed with high temperature secondary air. In this case, generally, the combustion of these fuels is effected in response to the total primary air amount and the secondary air in an amount corresponding to the difference between the theoretical combustion air amount and the total primary air amount. In this combustion, the temperature of the primary air is 60 to 80.degree. C., and the temperature of the secondary air is 800 to 1,000.degree. C., and thus the merits of the combustion vary depending on the primary air ratio (which refers to a ratio of the total primary air amount to the theoretical combustion air amount), the lower the primary air ratio, the higher the temperature of air used for the combustion, and as a result, the burning velocity increases, the fire point temperature becomes high, and good combustion occurs.
However, when the primary air ratio is decreased to make the combustion conditions better, disadvantages such as the ejection stream velocity decreases, and thus the mixing of the secondary air becomes insufficient, the burning velocity of the powder fuel and the liquid fuel becomes low, the fire point temperature decreases and the fuels are incompletely burnt, occur. For these reasons, in the conventional apparatus and method of mix-burning the fuels, the primary air ratio is usually about 20 to 25%, and it is practically difficult to carry out the mix-burning in a reduced primary air ratio at an increased burning velocity and at an increased fire point temperature. Also, in the conventional mix-burning apparatus and method, it is possible, to a certain extent, to adjust the position of the fire point by controlling the flow velocity ratio of the inner primary air straight streams and the inner primary air turning streams formed together with the straight streams. In practice, the above-mentioned control of one burner is difficult, and thus it is necessary to change the design of the inner primary air straight stream-ejection openings and the inner primary air turning stream-ejection opening, in response to the properties of the rotary kiln. In this case, when the inner primary air straight streams become too strong, the resultant combustion flame is in a narrow angle long flame form in which the fire point temperature is insufficiently low. When the inner primary air turning streams become too strong, the resultant combustion flame is a wide angle short flame in which the fire point temperature is sufficiently high and the flame becomes too wide, and thus the furnace wall is greatly damaged. In a worst case, the furnace wall is damaged.
In view of the conventional burning apparatuses and methods as mentioned above, there is a strong demand for an apparatus and method capable of forming a combustion flame in a narrow angle short flame form, of sufficiently raising the fire point temperature by using a powder fuel or a liquid fuel or using a powder fuel together with a liquid fuel, and of obtaining good combustion without damaging a furnace wall.